(1) Field of the Invention
The present invention relates to an electrode plate and a jig for use in plasma etching, which are used in the plasma etching of a wafer for production of a semiconductor integrated circuit.
(2) Prior Art
With the recent progress of the techniques for production of finer and more dense semiconductor integrated circuit, there has become increasingly important the plasma etching capable of forming a fine pattern on a wafer with high precision using parallel plate electrodes.
The material for the electrodes used in said plasma etching must have electrical conductivity, chemical stability, etc. As the material, there has been tried, for example, graphite. This graphite material is produced by mixing a coke or carbon powder with a binder (e.g., tar or pitch), molding the mixture and firing the molded material to graphitize it. Therefore, the material is difficult to produce so as to have a dense and homogeneous composition and, when fitted in a plasma etching apparatus as an electrode plate to conduct the plasma etching of a wafer, detaching of carbon particles from the electrode plate, occurs which accelerates the consumption of the electrode plate or stains the wafer (this hinders pattern formation).
Under such a situation, it is under investigation to use a vitreous carbon which has an isotropic composition and a high hardness, as an electrode plate for plasma etching. The vitreous carbon whose use as an electrode plate for plasma etching is currently being investigated, includes, for example, those produced as follows.
Japanese Patent Application Kokai (Laid-Open) No. 252942/1987 discloses an electrode plate for use in plasma etching comprising a high-purity vitreous carbon. According to the document, the vitreous carbon is produced by curing, in a plate form, a liquid furan resin, a phenolic resin, a mixture thereof, or a mixture of one of said liquid resins with a powder of same type curable resin and then firing the cured plate in an inert gas. According to the document, an example of such a vitreous carbon has a bulk density of 1.45 g/cm.sup.3, a porosity of 3%, a Shore hardness of 75, a bending strength of 580 kgf/cm.sup.2 and an elastic modulus of 2,430 kgf/cm.sup.2.
In the vitreous carbon produced by the above process, however, the pores formed during the molding step remain in the final product, or the volatile components are evaporated during the heat treatment step and thereby a large number of pores of large diameters (e.g., about 2 mm) are generated, as mentioned in Japanese Patent Application Kokai (Laid-Open) No. 285086/1991.
Also, Japanese Patent Application Kokai (Laid-Open) No. 285086/4991 discloses a vitreous carbon which is produced by basically the same process as in the above-mentioned Japanese Patent Application Kokai (Laid-Open) No. 252942/1987, that is, by subjecting a thermosetting resin (e.g., modified furan resin) to defoaming, curing the defoamed resin in a plate form and then firing the cured plate in an inert gas. This vitreous carbon as well has a porosity of 0.02-0.2%.
The above document makes no mention of properties other than porosity. However, the above vitreous carbon is presumed to have about the same properties as the vitreous carbon of the Japanese Patent Application Kokai (Laid-Open) No. 252942/1987 because it is produced using basically the same raw material and process as in the Japanese Patent Application Kokai (Laid-Open) No. 252942/1987.
Thus, in the conventional processes for production of vitreous carbon, water is generated by the dehydration reaction taking place in the curing step because a phenolic resin or a furan resin is used as a raw material; therefore, the formation of pores in the firing step for cured resin is inevitable. These pores, when the resulting vitreous carbon is used as an electrode plate for plasma etching, bring about an increase in specific surface area and reduction in oxidation resistance and strength.
Further in the vitreous carbons produced by the conventional processes, insufficient strength of the cured resin per se and the pores generated in the dehydration step pose a restriction in mechanical properties of said vitreous carbons.